Personally, I'd go with a solid mounted CD player in the glove box with a wireless remote or perhaps a Sony with a joystick remote and leave the factory radio in its place. I've done a few classics with custom kick panels with a nice 5.25 component set and custom rear decks with 6.5s or 6x9s depending on what the customer wanted. Don't forget the sound deadening material, a little will go a long way in a classic car. If you're going with a sub, then you may want alot.
I'm going to be building a custom fiberglass console for a friends 64 Malibu SS to hold an Alpine CD player and an Audio Control EQL. I think he wants me to mold some gauges in also, but we'll see. This could be an option for your
Stang but it will take away from the "classic" look.